Spectroscopic hole burning

At room temperature, non-photochemical spectral holes usually are filled in by flucmations of the surroundings on the picosecond time scale. This process, termed spectral dijfusion, can be studied by picosecond pump-probe techniques. At temperatures below 4 K, non-photochemical spectral holes can persist almost indefinitely and can be measured with a conventional spectrophotometer. The shape of the hole depends on the lifetime of the excited state and the coupling of the electronic excitation to vibrational modes of the solvent, both of which depend in turn on the excitation wavelength. Excitation on the far-red edge of the absorption band populates mainly the lowest vibrational level of the excited state, which has a relatively long lifetime, and the resulting zero-phonon hole is correspondingly sharp (Fig. 4.22A). The zero-phonon hole typically is accompanied by one or more phonon side bands that reflect vibrational excitation of the solvent in concert with electronic excitation of the chromophore. The side bands are broader than the zero- 

Each of the homogeneous lines in a vibronic absorption spectrum actually consists of a family of transitions between various rotational states of the molecule. The rotational fine structure in the spectrum can be seen for small molecules in the gas phase, but for large molecules the rotational lines are too close together to be resolved. 

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Polymer surfactant interaction has been examined by using sodium 2-(N-dodecyIamino)naphthalene-6-sulphonate as a probe. Solute-solvent interaction of free base phthalocyanine has been examined in both polyethylene and polystyrene by the effect of pressure on spectroscopic hole burning s Fluorescence has been used to indicate the onset of aggregation in water soluble polymers s interaction of pyrenylmethyltributylphosphonium bromide with single strand polynucleotides , and the interaction of indole compounds with synthetic polyelectrolytes. ... 

We are beginning to develop a detailed understanding of these methods (18,21,30,33,34,37-40,42,44,47-49), many of which are described in this book. We have recently demonstrated a series of novel nonlinear all-IR spectroscopic techniques (IR-pump-IR-probe, IR-three-pulse photon echoes, IR-dynamic hole burning, IR-2D spectroscopy), all of them utilizing intense femtosecond IR pulses, with the intention to develop new multidimensional spectroscopic tools to study the structure and the dynamics of proteins (30,31,41,42,50-53). We shall summarize in this contribution our work, its underlying principles, and its applications. 

Hole burning The photohleaching of a feature, normally a narrow range, within an inhomogeneous broader absorption or emission band. The holes are produced by the disappearance of resonantly excited molecules as a result of photophysical or photochemical processes. The resulting spectroscopic technique is site-selection spectroscopy. 

The first spectroscopic evidence of the presence of (S,H) and (S,D) centres in hydrogenated S-doped silicon was actually provided by Love et al. [155] in a study of spectral hole burning in the 2po and 2p lines of the (S, H)c2 and (S, H)c3 spectra inhomogeneously broadened in Si0.999Ge0.001 alloy samples. 

Within the last one and a half decades, it became possible to perform experiments directly on the atomic and molecular level. This came with the improvement of existing experimental techniques such as electron microscopy, where the resolution was increased to make single atoms visible [1] high-resolution spectroscopy of single ions or atoms trapped in a radio frequency field or in focused laser beams [2-4] and the spectroscopic isolation of single molecules in solids at cryogenic temperatures [5-7], which evolved from spectral hole-burning spectroscopy. 

Many tetrazines undergo an irreversible photochemical fragmentation with reasonable quantum yield even at very low temperatures. This fact has been used in several applications of the new spectroscopic technique of photochemical hole burning (HB). In the following sections brief descriptions of the principles of this method with references to applications to tetrazines, especially 3,6-dimethyl-1,2,4,5-tetrazine are given. 

For spectroscopic applications of multimode lasers one has to keep in mind that the spectral interval Ay within the bandwidth of the laser is, in general, not uniformly filled. This means that, contrary to an incoherent source, the intensity 7(y) is not a smooth function within the laser bandwidth but exhibits holes. This is particularly true for multimode dye lasers with Fabry-Perot-type resonators where standing waves are present and hole burning occurs (Sect. 5.3.4). 

Simply using a laser as a source does not take advantage of the full range of laser properties that are useful for spectroscopic purposes. The past decade and a half have witnessed the evolution of a host of laser based techniques which either have no readily identifiable equivalent in conventional spectroscopy or are difficult to carry out using the latter means. Two complementary techniques, fluorescence line narrowing (FLN) and hole burning, have found widespread uses in the spectroscopy of all phases principally as a form of homogeneous spectroscopy. 

Regardless, it should be clear that FLN and hole burning are complementary approaches and thus the method of choice depends in great part on the experimental resources available and the nature of the spectroscopic problem at hand. As noted above, there are almost as many variations to these two basic techniques as there have been spectroscopic applications. Techniques such as... 

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